A Biological Difference – Exercise Affects Boys and Girls Differently

Exercise Running at Sunset

The findings also indicate that boys with an increase in body fat percentage had an increase in sedentary behavior two years later.

A recent study finds that body fat percentage and amount of physical activity in girls are unrelated.

Physical activity provides numerous health benefits. However, physical activity affects boys and girls differently. A recent study analyzed the connection between children’s physical activity and body fat.

“We looked at the connection between objectively measured physical activity and the proportion of body fat in girls and boys,” says Silje Steinsbekk, a professor at NTNU’s (the Norwegian University of Science and Technology) Department of Psychology.

Rather than weight and height, the researchers assessed individuals’ body composition. They addressed questions such as: does greater physical activity result in a reduced proportion of body fat over time? Or is it possible that people who accumulate more body fat over time become less physically active?

Body fat and physical activity in girls are unrelated

The children were checked every two years by the researchers from the age of six to fourteen. They discovered that the degree of exercise had varied effects on the sexes.

“In girls, we found no connection between their physical activity and amount of body fat. Increased physical activity didn’t lead to less body fat in the girls, and body fat had no effect on changes in their physical activity,” says Tonje Zahl-Thanem, a former research fellow and first author of the article.

However, it’s different for boys. Their level of body fat impacts how active they are.

More body fat in boys results in less physical activity

“Increased body fat in boys led to less physical activity two years later, when they were 8, 10, and 12 years old,” says Zahl-Thanem.

With one exception, increased physical activity had no effect on changes in body fat.

“We found that boys who are more physically active when they’re 12 years old have a lower proportion of body fat when they’re 14. This wasn’t the case at an earlier developmental stage,” Steinsbekk says.

Several possible reasons for differences between the sexes

The study did not investigate the reasons for these differences, but the researchers point out that large bodies are heavier and require more exertion when exercising, which may explain why boys whose body fat increases become less active over time. But why isn’t this the case for girls?

“Here we can only speculate, but boys are generally more physically active than girls, so when boys reduce their activity level, the physical impact is greater,” Steinsbekk says.

We also know that children with large bodies are less satisfied with their bodies, and body dissatisfaction is associated with less physical activity in boys, but not in girls.

“Boys’ physical activity is probably even more competitively oriented than girls’, and more body fat makes it more difficult to succeed. Both of these conditions can help explain why increased body fat leads to less physical activity in boys, but not girls,” says Lars Wichstrøm, a professor in NTNU’s Department of Psychology and also co-author of the study.

It could also be that girls are more likely to maintain physical activity when their proportion of body fat increases because more attention is paid to girls’ bodies and appearance.

Body fat affects sedentary activity in boys

The researchers also examined the link between inactivity or a sedentary lifestyle and body fat. In the same way that they objectively measured physical activity, they also measured how long the participants were sedentary during the day.

“The results show that boys who had an increase in the proportion of body fat had a corresponding increase in sedentary activity two years later. This carried through all the age groups studied, from the age of 6 through age 14.

In other words, boys whose proportion of body fat increases become more sedentary.

For the girls, however, there was no link here either. The percentage of body fat did not affect their level of inactivity over time, and they did not become less active by gaining more body fat.

“In sum, we found a link between physical activity, sedentary lifestyle, and fat percentage in boys, but not in girls,” Steinsbekk says.

Trondheim Early Secure Study

The researchers used figures from the Trondheim Early Secure Study (TESS). They followed almost 1000 children at two-year intervals from when they were 4 years old. The participants are now 18 years old, and the eighth survey is underway.

In this study, the research group used data at five different times, when the participants were 6, 8, 10, 12, and 14 years old. The Trondheim Early Secure Study has provided data for a number of studies on children’s development and health.

Enter your journal: Reference: “Relations between physical activity, sedentary time, and body fat from childhood to adolescence: Do they differ by sex?” by Tonje Zahl-Thanem, Lars Wichstrøm and Silje Steinsbekk, 3 June 2022, International Journal of Obesity.
DOI: 10.1038/s41366-022-01156-6

6 Comments on "A Biological Difference – Exercise Affects Boys and Girls Differently"

  1. Another possibility is that there is a positive feedback in boys. That is, as they exercise less, they gain weight. As they gain weight, that causes them to exercise less, as they exercise less … Clearly, boys and girls tend to look different from those 60 or 70 years ago, at least in the US. Whether it is diet, amount of food, exercise, or “all of the above” may not be clear. But, considering the labor saving devices available to most, and the change in entertainment preferences (e.g. TV and video games), a lack of exercise probably plays an important role.

  2. Boys start building more testosterone for their body changes at puberty, and for being sexual as adults. That happens somewhere around 12 or 13 years old, which is where the change is first noticed. Instead of applying social values and attitudes, maybe we should look at the chemical changes in the bodies: increase of testosterone versus estrogen, since estrogen supposedly favors more fat on the body, or nutritional changes at puberty. The assumption that all girls are going to have the same social training and reaction to that training and therefore will all be different is a big stretch, and likewise for boys.

    If this study were done in Indonesia or Sri lanka, where the culture is different, would we get the same results? Possibly do it in countries where girls are expected to wear chadors, and might be less likely to exercise once they hit puberty as a result of social strictures that do affect most girls of the area.

    Did the study gathered data about the social architecture that the girls and boys were exposed to? Poverty or wealth? Different subcultures by language? Abuse creates its own subculture that is individualized often for particular family. Did they look at details of the cultures of the individual families? Whether there was great approval and validation, or whether they were subject to belittlement? Assuming that everyone in the study had the same cultural background when it comes to exercise in the case of girls is a big jump without data to support it.

    Especially considering that for boys the change happened at a particular age when their bodies are changing, we ought to look at biology more than social and societal and cultural impact.

    Since we know the age to look at, perhaps a shorter study looking at boys and girls ages 10, in case other cultures mature more early, through 16, might give contrast. Going to a culture where people of various genetic types are being exposed to different cultural norms than in this study, and comparing whether this is the same for people who have genetic differences versus cultural differences might be able to isolate whether this is a biological difference or a social one.

    But when you consider the age for boys at which the change happens, that says to me that we should focus on biology at least as much if not more than social impact.

  3. It could be possible that girls’ activity levels aren’t affected by weight because they are biologically programmed to carry extra reproductive weight, which aids them in having good physical health. This biological mechanism probably saved females from being cumbersome when living closely with the natural world.

  4. Why is there little acknowledgment that girls begin menses in this time period, bodies are preparing for fertility and growing life inside them, which all requires fat and calories!

  5. What an absolute joke of a “study”…is this what we’re calling “science” these days? Pathetic.

    Girls, don’t listen, as real science tells us, if calories in are less than calories out you WILL lose weight. And if you train your muscles, your body will be forced to burn fat. Period. People are looking for any excuse they can to tell Lizzo and the rest of the body positivity movement that it’s not their fault they’re unhealthy and obese.

  6. This article must be written a eighth grade drop-out. I’ve never read about a “study” in this general, run- on sentence format. They throw in Swedish- looking names to make it look valid. So, if you run a marathon a day and you are a female, you will not lose weight. Rediculous.

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